The present invention relates to a band-pass filter of a ladder type configuration using a surface acoustic wave (SAW) resonator in small-sized mobile communication devices such as mobile telephones, particularly to a SAW filter with attenuation poles having high attenuation in an attenuation band and low insertion loss in a pass band.
In recent years, mobile communication device terminals such as small-sized, lightweight mobile telephones have been developed rapidly. In accordance with this, the realization of reduction in size and of high performance in components to be used is being conducted. In response to this, RF (radio frequency band) components using a SAW filter are developed and used. Particularly, the SAW filter is developed actively to partly become commercially available because it is a device that greatly contributes to the realization of reduction in size of RF parts.
In order to enhance the performance of the mobile communication device terminals, a high-performance SAW filter is demanded which has lower insertion loss in a pass band and high attenuation in attenuation bands.
The high-performance SAW filter is used in components for branching, inputting and outputting signals in the mobile communication device terminals.
A traditional ladder type SAW filter forming the high-performance SAW filter is described in Band pass filter with low loss using SAW resonators, Sato, et al., IEICE Ronbunshi A, vol. J76-A, No. 2, pp. 245-252 (1993).
In addition, an attenuation pole filter shown in FIG. 12 added with an attenuation pole inductance L 330 is used as a high-performance filter.
FIG. 12 is a circuit diagram illustrating that a two-port circuit 380 of a traditional ladder type SAW filter formed of three series arms 310, 311, and 312 and two shunt arms 320 and 321 is serially connected to a two-port circuit 381 provided with an attenuation pole inductance 330.
As described in JP-A-10-163808, in the SAW filter with attenuation poles shown in FIG. 12, attenuation poles are formed in an attenuation band on the high frequency side of the pass band by the attenuation pole inductance 330, in addition to the characteristics of the traditional ladder type SAW filter. The attenuation poles can provide certain high attenuation characteristics in the attenuation characteristics on the high frequency side of the pass band.
In order to enhance the performance of the recent mobile communication device terminals, a high-performance SAW filter is demanded which has lower insertion loss in a pass band and high attenuation in the attenuation band on the low frequency side of the pass band. The high performance SAW filter is being investigated with the use of traditional bonding wire, but satisfying characteristics are not obtained yet under present circumstances.
In the traditional mobile communication device terminals such as mobile telephones, a SAW filter with attenuation poles having certain high attenuation characteristics in the attenuation band on the high frequency side of a communication band is shown in FIG. 12.
On this account, the circuit shown in FIG. 12 is used when attenuation poles need to be formed in the attenuation band on the high frequency side, and the traditional bonding wire is used for the attenuation pole inductance L 330 shown in FIG. 12.
In this example, traditional bonding wire is used as the inductance L to vary the frequency of the attenuation poles by this L value, and the characteristics are changed to realize certain high attenuation. However, satisfying characteristics cannot be obtained yet.
Furthermore, high attenuation characteristics are demanded in the attenuation band on the low frequency side, and attenuation poles need to be formed in the band. However, the traditional bonding wire is also used in the case. As a SAW filter with attenuation poles responding to this, a film is used in which the attenuation pole frequency of the attenuation band on the low frequency side of the pass band is adjusted by using the traditional bonding wire of the shunt arms in the ladder type SAW filter. However, required characteristics are not satisfied in this case as well. However, the traditional bonding wire is also used in the case. As a SAW filter with attenuation poles responding to this, a film is used in which the attenuation pole frequency of the attenuation band on the low frequency side of the pass band is adjusted by using the traditional bonding wire of the shunt arms in the ladder type SAW filter. However, required characteristics are not satisfied in this case as well.
In the mobile communication device terminals such as mobile telephones, a SAW filter with attenuation poles is desired which has high attenuation characteristics in the attenuation bands on the low frequency side and the high frequency side of the pass band. As a SAW filter with attenuation poles responding to this, a SAW filter with attenuation poles is used in which the traditional bonding wire is used as the inductance, an attenuation pole frequency is formed in the attenuation band on the low frequency side of the pass band, and the attenuation pole frequency is varied to provide high attenuation in the filter characteristics. However, the required characteristics are not satisfied.